Britain and the US have called for a ceasefire in Yemen “within hours” as they tried to seize on outrage caused by the killing of 140 people in a Saudi airstrike.

Fighting between Iranian-backed Houthis and the Yemeni government, which is supported by Gulf states, has lasted more than 18 months, far longer than the Gulf states expected.

The US secretary of state, John Kerry, said if Yemen’s opposing sides accepted and moved forward on a ceasefire then the UN special envoy, Ismail Ould Cheikh Ahmed, would work through the details and announce when and how it would take effect.

“This is the time to implement a ceasefire unconditionally and then move to the negotiating table,” Kerry said after a brief meeting with the British foreign secretary, Boris Johnson, and other officials in London. “We cannot emphasise enough today the urgency of ending the violence in Yemen.”

Kerry said he, Johnson and Cheikh Ahmed were calling for the implementation of a ceasefire “as rapidly as possible, meaning Monday, Tuesday.” Kerry and Johnson also met the Saudi foreign minister, Adel al-Jubeir.

On 8 October a Saudi air raid on a funeral killed 140 people and wounded 525 others, drawing severe criticism of the Arab coalition.

Cheikh Ahmed said the attack took place “amid significant progress in the long peace negotiations, and at a time when we were negotiating a durable accord.”

At the weekend Saudi Arabia admitted responsibility for the funeral attack and blamed incorrect intelligence and improper procedures. It said it was taking disciplinary measures, awarding compensation to families of the victims, and easing the air blockade that it enforces to allow the evacuation of the most seriously wounded for treatment abroad.

Britain’s Foreign Office said on Saturday it would take into account the Saudi investigation into the attack when deciding its policy on allowing arms sales to Saudi Arabia.

Britain knows it could be accused of hypocrisy if it condemns Russian backing for indiscriminate Syrian bombing but does not do the same in response to Saudi outrages.

Washington has accused Houthi rebels of firing missiles at US warships in the Red Sea on 9 and 12 October. The missiles fell short of their targets. On Saturday the US Navy said it was investigating another possible missile attack on a group of American warships.

The US military responded to the earlier attacks by hitting radar sites in territory controlled by the insurgents, defence officials in Washington said, in the first direct American action against the rebels.

An Omani aircraft evacuated those wounded in the funeral strike from Sana’a on Saturday. The flight also carried two American citizens who had been held in Yemen and were released after mediation by Oman. The US state department spokesman Mark Toner noted the “humanitarian gesture by the Houthis”.

The Omani aircraft also flew home to Sana’a rebel negotiators who, because of the air blockade, had been stranded in Muscat since the collapse of UN-brokered peace talks in Kuwait in August.